This invention relates to a method and a microprocessor-based apparatus for directly measuring the noise using a particular nuisance index associated with two physical parameters of the noise, namely the average energy level of the noise and the noise fluctuation level.
In the prior art methods and devices two different evaluation factors of the environmental noise are considered. The first factor is objective and relates to the physical measurement of the noise, the second factor is subjective and is bound to the difficulty of defining an appropriate nuisance index considering the reactions of people to the noise. While the physical measurement of the noise is currently carried out without excessive difficulty, the subjective aspect of the noise does not lend itself to one definitive solution. In order to provide a real evaluation of the nuisance caused by the noise, the objective parameters such as the noise intensity, the fluctuation, the level of the background noise, the characteristic of the single sound sources, as well as psycho-sociological factors and contingent character which can affect the response of the individual to the noise should be taken into consideration.
The noise parameter processing is effected almost by means of the physical data of the noise, the reliability of which is evaluated by calculating the statistical correlation factor between the objective noise data and the subjective response of people according to suitable evaluation criteria. Presently several indices for the evaluation of the nuisance caused by the environmental noise are provided (Schultz, T. J. Community Noise Rating, Appl. Science, Publ. Essex 1982). However, these evaluations suffer from the shortcoming that the calculation of the indices can be somewhat laborious being founded on a preliminary statistical analysis of noise samples. In addition, while it is current practice to measure the mean level of the noise, a clearly an precisely defined procedure for the noise fluctuation measurement is not yet available.
An index L.sub.DI (nuisance level of the noise) proposed by the present inventors some years ago (Cannelli G. B., Santoboni S., "Direct Measurement of Noise Nuisance by the New Index L.sub.DI, Applied Acoustics, 7, 47 (1974).) is correlated to two essential characteristics of the nuisance caused by the noise: the first one correlates to the sound intensity, the second one to the noise fluctuations. The L.sub.DI can be directly measured by means of an analog apparatus built by the same inventors (Cannelli G. B., Santoboni S., Apparatus for directly measuring the nuisance index L.sub.DI, Technical Report No. 34, IDAC (CNR), 1975). The index L.sub.DI as defined in the above publications has not represented the nuisance level of the noise with sufficient accuracy. It has been discovered that the index accuracy can be substantially improved if the negative noise fluctuation is disregarded. It has also been ascertained that the above mentioned analog apparatus does not provide an accurate measurement of the index L.sub.DI because of certain non-linearities introduced by the automatic gain control in the amplification of the input signal.
Additionally the analog apparatus does not allow the measurement of index L.sub.DI to be effected by a single operation a period of time greater than some tens of seconds due to problems caused by the drift of the analog integrators.
Accordingly, it is desired to provide a method and apparatus which allows measurement of the index L.sub.DI over a relatively long period of time.